An emergency power unit (EPU), also called an emergency power supply unit (EPSU), is a device for energy production in case of failure of the primary systems.
For example, the EPU used in the F-16 aircraft employs gas turbine powered by the engine bleed air or by hot gases generated by catalytic decomposition of hydrazine, which then drives the emergency electricity generator and the emergency hydraulic pump. On some commercial aircraft, such as the Airbus A330, a ram air turbine is used for emergency hydraulic power.
Elevators often use battery-based EPUs to facilitate the descent of the elevator cabin to a safe position when the building power supply fails.
Uninterruptible power supply units and generators can be referred to as EPUs.